Discourse on Thinking

Needless to say heavy duty Heidegger read.
It's about Gelassenheit (releasement) and the introduction which is as long as the original text itself is very informative.
Heidegger is light on his usual new vocabulary on this one which makes it a little easier to read.
Read the book to write a class (Philosophy of Technology) paper about Gelassenheit and found this book to be the core of Heidegger's thought on this topic.